Phase modulation receiver and automatic frequency control



9', 1940-1. M. G. CROSBY PHSEv MDUIJTION- RECEIVER AND AUTOMATICFREQUENCY CONTROL Filed Feb. 10, 1957 5 Sheets-Sheet l (IHREM/70N lsF/[TER INVENTOR M URRAY G. CROSBY BY w ATTORNEY M. G. vCROSBY July 9,1940.

PHASE MODULATION RECEIVER AND AUTOMATIC FREQUENCY CONTROL 5 Sheets-Sheet2 Filed Feb. l0, 1957 INVENTOR M U RRA?. CROSBY @9876543 kbn ATTORNEY M.G. cRosBY 2,207,691 PHASE MODULATION RECEIVER AND AUTOMATIC FREQUENCYCONTROL Filed Feb. l0. ""f'* 5 Sheets-Sheet 5 July 9, i940.

July 9, 1940. M. G. CROSBY 2,207,591

PHASE MODULATION RECIVER 'AND AUTOMATIC FREQUENCY CONTROL Filed Feb. lO,1937 5 Sheets-Sheet 4 I @Hmmm/R005: 106 @1mm/7mm: 5 i

MURRAY G. Rosff,

ATTORNEY l,

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July 9, 1940.

Patented July 9, 1940 PATENT OFFICE PHASE MODULATION RECEIVER ANDAUTOMATIC FREQUENCY CONTRGL Murray G. Crosby, Riverhead,I N. Y.,assignor to Radio Corporation of America, a corporation of DelawareApplication February 10, 1937, Serial No. 124,967

13 Claims.

The present invention concerns a novel method of and means fordemodulating wave energy modulated in phase at signal frequency and anovel method of and means for producing differential potentialscharacteristic of the deviation of the mean frequency of the receivedwave energy from its assigned frequency and utilizing said potential tocorrect forsaid deviation.

This disclosure describes a phase modulated Wave receiver of the typewherein to receive phase modulation a frequency modulation receiver isused in conjunction with a correction circuit which converts thefrequency modulation receiver output into a phase modulation output.

' Intead. of the usual audio or modulation frequency correctioncircuitused for this purpose, an intermediate frequency correctioncircuit is employed in the present system so that the phase modulationon the wave is corrected to frequency modulation before being impressedon the frequency modulation receiver portion of the system.

In the prior art, a receiver of the nature of the type herein disclosedconsisted of a frequency modulation receiver with an audio frequencycor`- rection circuit inserted in the receiver output leads. CrosbyUnited States application #618,154 led June 20, 1932, describes such afrequency modulation receiver for reception of phase modulations. Owingto the inherent difference between frequency and phase modulation, ifphase modulation is received on a frequency modulation receiver withoutthe aid of acorrection circuit, the receiver output is distorted in therespect that the output is directly proportional to thefrequency ofmodulation. Thus, a 100 cycle modulation would have an output of one,200 cycles an output of two, and so on. That this is true is shown inthe following analysis:

It is well known that the expression for a phase modulated Wave is givenby e=E sin (wt-l-qb sin pt) (1) where w is 21r carrier frequency, fc; qiis the phase deviatic 1 with modulation, and p=21r the modulationfrequency fm. The frequency of the w .ve of Equation 1 is of phase orFrom (3) it can be seen that the effective frequency deviation of aphase modulated wave is directly proportional to and fm. Consequently,

(Cl. Z50-20) the output of a frequency modulation receiver, receivingphase modulation, would be directly proportional to the modulationfrequency fm, in addition to the depth of modulation 1. Therefore, inorder to remove this distortion so that the receiver reproduces allmodulation frequencies with the same amplitude, a correction circuitmust be inserted which applies a correction inversely proportional tothe modulation frequency. In the receiver of Crosby United Statesapplication #618,154 filed June 20, 1932, this correction was applied inthe output audiofrequency circuits.

In the receiver of the present disclosure, the correction is applied tothe modulated wave in such a manner that the side bands on both sides ofthe carrier are attenuated an amount directly proportional to theirfrequency spacing from the carrier. In this manner, the depth of phasemodulation on the wave is made inversely proportional to the modulationfrequency. Consequently, the directly-proportional-to-frequency output,produced by the reception of phase modulation on a frequency modulationreceiver is compensated for. This type of correction will not bebothered by the distortion which is encountered in the audio correctedback-to-back type of phase modulation receiver. In receivers of theaudio corrected back-to-back phase modulation type, reception is marredby distortion similar t that effected by carrier fading on amplitudemodulation and to about the same extent as the distortion normallyencountered on amplitude modulation. In the present receiver whereincorrection is accomplished in the intermediate frequency circuits of thereceiver, an exhaulted carrier effect is obtained. This eliminatesthefading distortion which is encountered with the audio corrected type ofreceiver. The receiver of the present invention in this respect actssomewhat like a Stenode Radiostat except that in the present case nocorrection of the audio frequency potentials in the output circuit isrequired as in the case of the Stenode. For Aan example of a receiver ofthe type wherein the side band attenuating effect is compensated for byan audio frequency voltage or potential correcting network see RobinsonUnited States Patent #1,821,032, dated September l, 1931.

A second novel feature of the present invention resides in the use ofdiode detectors for producing differential currents for automaticfrequency control purposes. Heretofore, the potential drop acrossresistances connecting the anodes of a pair of differential detectors inpush-pull relation has been used to control the frequency of the localoscillator beating with incoming frequency or phase modulated waveenergy to proplication #616,803 led June 13, 1932, nOW Patent#2,065,565. `In the present system, the detector inputs are energized ina mannersimilar to the prior system but the present system differs fromthe prior system in that diode detectors are used and are used in anovel manner to give the differential potentials necessary for frequencycontrol purposes. 'I'he cathode of one diode rectifler is groundedthrough a resistance, while the cathode of a second diode rectifier isconnected directly to ground. The anode of the first diode is connectedto ground by an alternating current circuit. The anode of the seconddiode is connected through an alternating current input circuit and asecond resistance to ground. The ground connections are optional, theessentiaal feature being relatively fixed potentials at particularpoints in the circuit. Both resistances are in series with an input andoutput circuit so that opposing potentials appear at the terminals ofthe resistances remote from ground or cathode or point of xed modulationand radio frequency potential and the state of equality of theseopposing potentials indicates the correctness of tune of the receivedwave with respect to the intermediate 'frequency channel. Obviously theground connections are not necessary, the essential feature being theproviding for use of opposed potentials at the impedances.

In describing my novel invention in detail, reference Will be made tothe attached drawings, wherein:

Figure 1 illustrates schematically and by rectangles the essential unitsor elements of a phase modulation Wave receiver arranged in accordancewith the present invention;

Figure 2 illustrates somewhat more completely the receiver of thepresent invention and in addition illustrates a novel diode rectifiersystem and its association with a local oscillator for beating with thereceived wave, this being an additional novel feature of applicantsinvention;

Figure 3 illustrates a modified form of differential diode detectorsutilized for producing control potentialsto tune the local oscillator;

Figure 4 illustrates by curves and graphs the characteristics which thecorrection filter and sloping filter and a combined filter having thecharacteristics of both filters used' in the prior figures must have; i

Figure 5 illustrates a highly selective filter utilized in the priorgures to attenuate the remote side frequencies to thereby convert phasemodulation to frequency modulation;

Figure 6 illustrates one type of filter circuit having a slopingcharacteristic for converting frequency modulations on the intermediatefrequency wave to amplitude modulations and includes also demodulatingmeans;

yFigure 7 illustrates graphically the character of the sloping filtersof Figure 6 and their relaf tionship which produces the demodulation ofthe Aconverted phase modulations produced in 5 and compensation ofamplitude changes caused by static or other undesired signals;

Figures 8 and 9 illustratefilters of the neutralized type which maybeadjusted for. a small degree of o-neutralization to produce the combinedeffect of a highly selective filter and a sloping filter; while Figures10, 11a, 11b, and llc are curves illusl trating the characteristics ofthe filter circuits of Figures 8 and 9 as used in my novel receivers.

The circuit of Figure 1 shows a blocky diagram of the essential elementsof the receiver.' Units I, 2, 3, and 4 are the elements of asuperheterodyne receiver which, although it is not an absolute necessityfor this receiver, facilitates 'the design of the correction lter ofunit 5. This filter takes the form of a sharply tuned circuit which issharp enough to cut side-bands. It may be a tuned amplifier with orwithout the application of a negative resistance effect or, morecommodiously, it may be a crystal filter such as is shown in Figure 5,which will be described hereinafter. Figure 8 when the same iscompletely neutralized. After the phase modulated intermediate frequencyenergy is passed through the correction circuit of unit 5, Where, byvirtue of the decrease in depth of modulation of the higher modulationfrequencies, it is converted to a frequency modulation, accompanied byrelative exhaltation of the carrier Wave, it is passed to the slopingfilter of unit 6. This sloping filter is of the normal frequencymodulation receiver type which has a minimum output at one edge of theband and increases linearly with respect to frequency change to amaximum at the other edge I may also use the filter of of the band. Asimple ofi-resonate tuned circuit will suiiice here. Due to the slope ofthis iilter,`the frequency change is converted to an amplitude change sothat the energy fed to detector and audio frequency amplifier unit 'l isamplitude modulation which is detected in the conventional way. Theoutput of the receiver is taken from pli-ones or other utilizationdevice 8. Thus, the energy has been heterodyned, converted to frequencymodulation, converted to amplitude modulation, and then detected. In apreferred embodiment, the sloping filter of unit 6 and demodulator ofunit 'l are combined as shown in Figure 6.

On the curve sheet of Figure 4, the dotted line curve A shows therequired correction characteristic whichl would correct phase modulationto frequency modulation for the speech band of audio frequencies, 250 to3000 cycles.

output terminals of the sloping filter will be the y product of the twofilter characteristics such as -.is shown by the full line C of Figure4.' Hence,

the correction and sloping filters may be replaced as illustrated inFigure 2 by a single filter which hasta characteristic as shown by C ofFigure 4. Such a characteristic isfairly easy to obtain. A crystalfilter of the neutralized type may be adjusted to this characteristic byadjusting the neutralizing condensers for a small degree'ofoflneutralization. Such a filter circuit has been shown in Figures 4 and6 of my United States applicationv #47,933 iiled October 5, 1936, nowPatent #2,085,008 and in Figure 8 of the present disclosure. The sharplytuned shielded crystal filter of Figure 7 of Crosby United Statesapplication #616,803 filed June 13, 1932, now Patent #2,065,565 may alsobe adapted by varying the completeness of shielding. Of course, thefilter of Figure 8 of the present application may be used when the sameis off-neutralized. A tuned circuit filter may be giventhischaracteristic by choosing tuning elements whose effectiveresistance varies with frequency.

The-circuit of Figure 2 shows a more complete circuit arrangement ofthis type of intermediate frequency corrected receiver and frequencycontrol means. In this circuit any wave energy pickup means suppliesphase modulated wave energy to a radio frequency amplifier I coupled atits output to a detector 2 which is supplied by oscillatory energy fromoscillator 3 by a pick-up means 24 coupled to an oscillator circuit 22.The beat frequency output from 2 is supplied to an intermediatefrequency amplifier and amplitude limiter 3|. Automatic tuning controlis applied to the receiver in order to hold the receiver in tune withthe sharp correction lter in unit 9. The output of 3| is fed in phaseopposition to the automatic frequency control diode detectors I I and I4via transformer I0, and also to the combined or separate correction andsloping filters in unit 9. The output of unit 9 is fed in phase to theautomatic frequency control detectors II and I4 via transformer II. Morein detail intermediate frequency energy modulated in phase is impressedon the primary winding P of transformer I0 which is tuned tosubstantially the mean frequency of the said energy by means of thecondenser C. This energy is induced in the two secondary windings S` andSI and applied to the anodes of diode detectors II and I4 respectively.Corrected and filtered energy is impressed from the output of 9 on theprimary winding IP of transformer Il which primary is tuned by thecondenser CI to substantially the mean frequency of the energy passed by9. Voltage is induced from primary winding iP on secondary windings ISvand ISI and supplied therefrom to the anodes of diode detectors II andlli, respectively. The adjustments of these circuits are such that thevoltages supplied from 3| and 9 to the respective rectifiers are inphase displaced relation. As an example, if the voltages supplied by Sand SI are in phase opposition onthe anodes of II and Ill, the voltagessupplied by IS and ISI may be displaced in phase by similar amounts Withrespect to the aforesaid voltages supplied by SI and S. Obviously eachdetector is, due to these two phase displacedY voltages, supplied with aresultant, the amplitude of which changes when the phases of one or bothof the supplied voltages changes. The variations in the resultants aredetected in II and I4 and appear in I2 and I3. The automatic frequencycontrol controlling energy is fed from points on resistance R1 R2 tomodulator tubes I8 and I9 via time constant circuits I5 and I6. Sinceresistor I2 is in the cathode circuit of rectifier I I and resistor I3is on the loW potential side of the secondarywindings of thetransformers I0 and I1, the potential drop due to rectified directcurrent in I2 will be of opposite polarity with respect to the potentialdrop in I3. 'That is, an increase of alternating current voltage to theinput of diode I I will cause resistor I2 to pass a positive voltagethrough R1 to the control electrode of modulator I8, while an increasein alternating current voltage to the input of diode I4 will cause I3 topass a negative voltage through Rz to the control electrode of modulatorI9. Consequently, as the combination of the filtered and unfilteredvoltages, in transformers I0 and I'I, produce a high resultant voltageto one diode and a low resultant voltage to the other diode (by virtueof any off-tune condition which causes the intermediate frequency not toequal the mean frequency to which 3| is tuned) according to theprinciples described in Crosby United States application #616,803 filedJune 13, 1932, now Patent #2,065,565, one diode will bias its modulatorI8 or I9 more negative and the other diode will bias its modulator I9 orI8 less positive. In this way, the resultant effect of the two diodesI`I and I4 is in the same direction so that both .modulator tubes arebiased in the same direction for a given unbalance of alterhatingcurrent voltages to the diodes due to an oif tune condition. When thegrid biases of modulator tubes I8 and I9 are varied, in a directiondetermined by the direction of deviation of the Wave frequency from thedesired Wave frequency, by the automatic frequency control detectorenergy passed by time control circuits I5 and I8, the plate resistancesof the modulator tubes I8 and I 9 vary. These plate resistances are inseries with capacity 2I which is placed across the tuned circuit 22 ofhigh frequency oscillator 23. Plate potential for both modulator tubes I8 and I9 is supplied by a source of direct current +BI through radiofrequency choke 20. Hence, as the plate resistances of modulators I8 andI0 vary, the effective capacity of 2| is varied so that the tuning ofthe high frequency oscillator is varied. In this manner when the signalis off tune with respect to the intermediate frequency amplifier 3i andthe correction filter in 9, the differential combination of alternatingcurrent voltages in transformers IU-and I'I causes the modulator tubesI8 and I9 to change the tuning of the high frequency oscillator 23 andbring the intermediate frequency energy back in tune with theintermediate frequency amplifier 3| and the correction filter 9.

The output of correction and sloping filter 9, which is phase modulationconverted into amplitude modulation, is fed, via transformer 25, todiode 26 for rectification. Filter 21 removes the intermediate frequencyenergy from the output and makes the potential drop in 28 characteristicof the phase modulation on the received wave available for amplificationin 29, the output of which is available at jack 30.

If desired, the automatic frequency control detector circuit of Figure 3may be applied to this type of receiver. As will be seen thisarrangement is similar in many respects to that shown in Figure 2.However, in Figure 3, the rectified voltages from the automaticfrequency control detectors are combined to charge condenser C throughresistors R1 and R2, the terminals of which are here connected4 togetheras Shown. Thus, for the condition of balance, the positive voltage fromone diode resistor neutralizes or compensates the negative voltage fromthe other diode resistor so that a mean nominal resulting potential isproduced and condenser C is not charged. An unbalance of the detectorcurrents, causediby an off-tune condition in the received wave oroscillator 23 or both, causes the charge of condenser C to be changedand the bias of the modulator tube 32 to be changed in a manner to varythe effective capacity of condenser 2| in the proper direction tore-tune the circuit 22 and the entire receiver circuit. Due to thesimilarity between the remaining portions of Figures 2 and BlandA theuse of like reference characters for similar parts, no furtherdescription of Figure 3 is necessary.

The highly selective lter circuit used in the correction filter unit 5of Figure 1 and the correction circuit of combined unit 9 of Figures 2and 3 may comprise any circuit having the desired characteristics.4 Forexample, the circuit of unit 5 may be of the crystal filter type asillustrated in Figure 5. In this arrangement wave energy the controlgrid and cathode respectively, of an amplifier or relay tube 62. Therelay tube 62 is supplied with biasing potential by way of a resistance68 and a source of potential 61 and with anode potential by way of asource 65 andthe primary of an output transformer 66. 'Ihe selectedenergy may be impressed from the secondary of transformer 66 on theinput electrodesof the sloping filter circuit and from the outputthereof in phase on the input electrodes of the diodev detectors and I4and also on the input of the tube 26 supplying the signal potentialsresulting from demodulation to the utilization circuit.

'I'he sloping lter utilized in the unit 6 of Figure 1 or in the units 9of Figures 2 and 3 may be of any type and may for example, be asillustrated in my United States applications #618,154 led June. 20, 1932and #703,770 filed December 23, 1933, now Patent #2,060,611.

In the circuit of Figure 1, the sloping filter and detector units 6 and1 preferablyv are replaced by a back-to-back frequency modulationdetector.

'I'he circuit of Figure 6 shows such a detector unit.

The intermediate frequency output is fed from the correction filter 5 ofFigure 1 to the terminals XX of Figure 6. Coupling tubes 10 and 1| havesloping filter circuits L1C1Co and LzCzLo, respectively, in their platecircuits. These filters produce characteristics such as shown by curvesI and 2 of Figure 7. Multiple resonance is formed by the circuits sothat for the case of the L1C1Co circuit a characteristic llike curve Iof Figure 7 is produced by virtue of a parallel resonance at frequencyF2 and series resonance at frequency F1. That is L1C1Co is adjusted toparallel resonance at a frequency F2 which is above the mean frequencyof the wave to which the transformer T is tuned and series-resonant atthe frequency F1 which is below the mean frequency to which thetransformerT is tuned, the latter frequency being represented by Fc inFig. 7. Under these circumstances, a maximum voltage will be producedacross R3 when wave energy is impressed on the filter of a frequency ofthe order of F2 and this voltage willv decrease to a minimumas the saidimpressed frequency decreases to-'a frequency of the order of F1. Thelter L2C2Lo is series-resonant at the frequency Fzrvand parallelresonantat the frequency F1 so -that it has a characteristic similar to butopposed to the i-llter L1C1Co. The characteristic of the filter LzCaLomodulation (bythe push-pull connection).

is shown at2 in Fig. 7. Filter LzCzLo produces the opposite slope shownby curve 2 by interchanging the points of parallel and series resonance.

As shown in Figure 6 the wave energy to be converted may be impressed onthe input of radio frequency transformer T and from the input of T inphase on the control grids G1 of tubes 10 and 1|. These tubes may be ofthe suppressor grid type or of any other type, having the desiredcharacteristics. The anodes A of tubes 10 and 1| are connected as shownwith the filter circuits of sloping characteristics. 'I'he circuit Co,C1 etc., is connected at the high poten- .tial end of the radiofrequency choke connected with 10, while the circuit LoLz etc., isconnected directly to the anode A of tube 1|. The high potential ends ofthe series circuits Co etc., Lo etc., are connected as shown to thecontrol grids 13 and 14 of a pair of detectors 12 and 16 operating asdiodes. The anodes of tubes 12 and 16 are connected to a source ofpotential, while the cathodes are connected together and/or to ground byhigh impedances R1 and R2. Coupling condensers C connect the highpotential ends of the impedances R1 and R2 to the control grids ofamplifying tubes 80 and 8|, the anodes of which are coupled in parallelor in push-pull by switch S and to an output circuit 83. Self-biasoperation is obtained'in tubes 80 and 8| by connecting the cathodesthereof to the grids by way of resistances 11 and 18.. 'I'he seriescircuits Co etc., Ln etc., are shunted as shown by high impedances R3and R4.

The detectors of this circuit, tubes 12 and 16, are of theinfinite-impedance'diode type. 'I'his type of detector is used in orderto obtain a linear detector that does not place a low resistance load onthe filter circuits and thereby cause their slope to be decreased. Thedetected outputs from the detectors are taken from the cathode resistorsand coupled to the grids of amplifier tubes 86 and 8|. The transformerin the plate circuits of tubes 86 and 8| may be switched by S to eitherthe parallel or push-pull connection sothat amplitude modulation may bereceived (by the parallel connection) as well as phase and frequency Ifamplitude modulation were received, the correction filter would beswitched out of the receiver. In accordance `with the principlesdescribed in my United States application #114,894 filed Dec. 9, 1936,now Patent #2,154,398, either of the sloping filters might be replacedby a fiat-top im` l pedance coupling giving a characteristic as is shownbythe dotted curve 3 of Figure 7. This could be done, for instance, byeliminating the inductance Lo and making the grid resistance of tube 16low enough to dampr the tuned circuit LzCz to make it a broad impedancecoupling.

Of course, the two filters of Figure 6 may, if

desired, -be replaced by a simple resonant circuit ing' the combinedcharacteristics may comprise as illustrated in Figure 8 a piezo-electriccrystal 94 connected as shown at one terminal with the secondary winding9i of an input transformer 90 having its primary winding coupled withthe output of the intermediate frequency amplifier 4 of Figure 1 orintermediate frequency amplifier and amplitude limiter 3| of Figures 2and 3. The other terminal of the secondary winding is connected as shownby way of a neutralizing condenser Cn tothe other terminal of thepiezoelectric crystal 94. This terminal of the crystal is connected asshown to the control grid of a tube 95, the cathode of which isconnected by a biasing resistance 96 to a leak resistance 96 and to apoint on 9|. The anode of tube 95 is connected to an output circuit 99which may be coupled to I1 and 25 -as shown in Figures 2 and 3, or to `Ias shown in Figure l. The output of the crystal comprises the energypassed by the crystal and also any unneutralized energy passed by thecrystal holder.

The energy at the output of the crystal may be impressed directly on theprimary winding IP of transformer I'I, the in-phase secondaries IS andrISI of which are connected with the diode detectors and on the input ofthe utilization circuit or may be impressed on the input electrodes ofthe amplifier 95 as shown and from the output electrodes of said tubeson the said windings of Il and from thence to the diode detector systemand impressed on 25 and from thence to a utilization circuit. Thecircuit of Figure 8 as to the crystal effect alone on the wave passedwill have a characteristic as shown by curve A of Figure 4. As to thecrystal and holder effect the circuit wil be resonant at one side of thecarrier or crystal frequency as described herein later and will producea resultant effect as illustrated by curve C.

The filter of Figure 9 which is in some respects similar to the filterof Figure may also be used to obtain the side band attentuating andsloping filter effects. This filter comprises a shielding box enclosinga piezo-electric crystal Ibi, capacity members and crystal electrodes II2 and IM mounted on opposite sides of a movable shield member 99carried by a movable member H9. The movable member III) is shown as ashaft mounted for rotation and controlled by a member it. The input tothe filter is between a connection through an insulating member I02 andthe shield while the output is between II5 and the shield. Morespecifically, energy to be filtered is supplied by a tuned circuit to aconductor passing through the shield and insulated therefrom byinsulator |02 and a point of ground or shield potential the saidconductor and ground point in turn being connected to crystal electrodesas shown. The output from the filter is taken from a second pair ofelectrodes as shown connected to a contact passing through the shieldbox and insulated therefrom by insulator II5 and to the shield Theposition of the shielding member |08 with respect to the capacitymembers H2 and I I4 determines the shielding between the crystal outputand input and consequently the neutralization of the circuit.

I Will now describe, the two circuits of Figures 8 and 9 which aresuitable for the functions of either attenuating the side bands alone orfor attenuating the side bands as Well as producing a sloping filter.T-'he first of these is the neutralized crystal filter circuit of Figure8 of this disclosure. When the neutralizing condenser, Cn, of thiscircuit is adjusted for complete neutralization so that thecharacteristic of Figure 10 is obtained, the conditions are proper forthe attenuation of the side bands alone. This adjustment of the filterwill attenuate the side bands so that the phase modulation is correctedto fre- 5 quency modulation for subsequent conversion to amplitudemodulation by a sloping filter following this crystal filter. Withoutneutralization, or with over-neutralization, a characteristic such as isshown in Figure lla will be obtained. HOW- 10 ever, as the neutralizatonis made more complete, the dip point Fd in the characteristic of Figurella is moved farther away from the carrier frequency, Fc as shown inFigures 11b and llc. Thus, vthis dip point may be set outside of 15 theside band channel by an adjustment of the neutralizing condenser.Consequently, a filter may be effected with one side dipped slightly sothat it is lower than the other as shown in Figure 4, curve C. In thisway a slightly-neutralized crystal filter acts as a combined correctingfilter and sloping filter to convert phase modulation directly intoamplitude modulation for detection.

The device of the attached Figure 9 is a crystal filter of the shieldedtype which may be adjusted to the same type of characteristics as theslightly-neutralized type. The shielding takes the place of theneutralization. Hence by varying the shielding, the effect is the sameas though the neutralization were varied. That is, the shielded crystalholder of Figure 9 has a grounded shield plate Hi8 between the crystalelectrodes and is carefully lconstructed so that the capacity of thecrystal holder may be completely shielded out. In order to make possiblea variability of the shielding effect, the metal shield plate I08 placedbetween the crystal electrodes is mounted on a rotatable shaft III) sothat the plate may be moved out from between the electrodes II2. and II4. The electrodes are bent to such a shape that a relatively largeamount of capacity will be obtained between the crystal electrodes whenthe shield plate is removed: this insures that a capacity will bepresent to cause under-neutral# ization when the shield plate is movedaway. 5 Thus by rotating the knob |96 on the shaft where the shieldplate is mounted, the crystal filter characteristic may be changed fromthe completely shielded or neutralized characteristic of Figure l0 tothe slightly-neutralized type of 50 characteristic similar to Figure llcbut with the dip outside of the side band channel. Consequently thistype of filter is also capable of the adjustment which produces eitherthe single function of attenuating the side bands or the dual functionof attenuating the side bands and effecting the sloping filter.

In order to clearly disclose the operation of filters 8 and 9 furtherdescription of what takes place whenl the crystal filter isoff-neutralized or off-shielded will be given. Reference will be made toFigures 11a, 1lb, and llc which helps to explain the operation of thefilters. The curve of Figure 11a shows the characteristic of a crystalfilter which is un-neutralized. Inspection of this curve 55 shows thatat the series resonance frequency of 'the crystal a high output isobtained at the carrier frequency, Fc. At the parallel resonancefrequency obtained by the inductive effect of the crystal and thecapacity of the crystal holder, the filter output has a dip at thefrequency Fd. This dip point in the characteristic may be moved fartheraway from the carrier frequency Fc as shown in the curve of Figure 11b,by increasing the neutralizing condenser Cn of Figure 8 towards 75 theconpletely neutralized point, or by moving the shielding member '|08 ofFigure 9 towards the completely shielded point. By a further increase inthe degree of neutralization or shielding, the dip point Fd may be movedcompletely out of the working' channel (the working channel is betweenthe frequencies Fi and F2) as is shown inthe curve of Figure 11e. From astudy of the curve of Figure 11e, it can be seen that the eiect ofplacing this dip point Fd just outside of the workingchannel is to lowerthe output of the ilter on one side of the carrier frequency withrespect to the output on the other side of the carrier frequency.Consequently the desired characteristic of curve C of Figure 4 isobtained.

'Ihe sloping filters and detector umts of Figure 1 may be replaced byany type of frequency modulation receiver such as the ones described inCrosby United States applications: #618,154, filed June 20, 1932;#703,770,.led December 23, 1933, now Patent #2,060,611, issued Nov. 10,1936; #114,894, filed December 9, 1936, Patent #2,154,-

' 398, issued April 11, 1939. In this manner, the

back-to-back balance effect of the frequency modulation reception couldbe obtained.

, The filters of Figures 8 and 9 may be used in the unit 5 of Figure 1in place of the iilter of Figure 5 in which case they are completelyneutralized or shielded to have a characteristic as illustrated inFigure 10.

In any of these receivers, the amplitude limit- .trol (AFC) which isapplied may be of the type wherein thel differential detector energyoperates a relay controlling a motor drive to rotate a tuning condenseras is described in Goddard United Statesapplication #11,915 led March20, 1935.

1. In a modulated wave receiver, an amplitude modulated wave demodulatorhaving an -input circuit, circuits including a piezo-electric crystalforming a filter having a highly selective characteristic and a slopingcharacteristic, said lter having an input and an output, means forimpressing modulated energy on the input of said filter, and means forapplying modulated wave energy from the output of said lter on 'theinput circuit of said demodulator.

2. In means for demodulating wave energy modulated in phase at signalfrequency, a detector, a lter circuit having a highly selectivecharacteristic anda sloping characteristic, said filter circuit havingan input and having an output coupled to said detector, and means forimpressing phase modulated wave energy on the input of said ltercircuit.

3. The method of receiving and demodulating Wave energy modulated inphase and comprising a carrier and side band frequencies which .includesthesteps of, attenuating side band frequencies remote from the carrierfrequency to thereby distort the said phase modulations on said waveenergy to obtain modulated wave energy which has the characteristics offrequency modulated wave energy, simultaneously converting saidfrequency-modulated wave energy into wave energy correspondinglymodulated in amplitude and demodulating said amplitude modulated waveenergy.

4. The method ,of demodulating wave energy modulated in phase andcomprising a carrier and side band frequencies which includes the stepsof, attenuating side band frequency remote from the carrierfrequenciesto thereby distort the said phase modulated energy so that ithas thecharacteristics of frequency modulated energy, amplifying theresulting energy and imposing frequency discriminations on the amplifiedenergy to convert said frequency modulation into amplitude modulation.

5. In a phase modulation receiver system, a tunable circuit including atuning reactance and tuning means therefor, a phase modulated waveamplifier coupled to said tuning means, said wave amplifier having anoutput a highly selective filter` and having an output from vwhichamplied Wave energy may be derived, a pair of diode detectors eachhaving an anode and a cathode, an alternating current circuit and a'highimpedance connected between the anode and cathode of one of said diodedetectors, a connection between a point on said high impedance remotefrom the cathode of said diode detector and ground, an alternatingcurrent circuit including a high impedance connected betwen the anodeand cathode of the other of said diode detectors, a connection betweenthe cathode of said last named diode detector and ground, circuitscoupling said alternating current circuits to the output of saidamplier, additional impedances connected in shunt to said highimpedances, a coupling tube having an anode, a cathode, and a controlgrid, a circuit connecting the anode of said coupling tube to saidtunable circuit, a circuit coupling the control grid of said couplingtube to a point on said additional impedances, and a connection betweenthe cathode of said coupling tube and ground.

'7. In a wave energy demodulating means, a

source of wave energy having an output, a load circuit having an input,wave filtering means having a selective and sloping characteristiccoupled at its input to the output of said source of wave energy, saidwave filtering means having an output and means for rectifying theoutput of said filter and impressing the rectified energy onV the inputof said load circuit without materially loading said filter comprising adetector tube having an anode, a grid, and a cathode, an impedanceconnected between said cathode and a point of fixed potential, means formaintainingA said anode at a fixed positive potential relative to saidpoint of xed potential, a circuit coupling said grid and point of xedpotential to the output of said filter, said detector tube being of veryhigh input impedance, and a circuitconnecting said load circuit input inshunt to said impedance.

8. In a system for demodulating Wave energy modulated in phase at signalfrequency, means energy to distort the phase modulated wave so that ithas characteristics of frequency modulated wave energy, a' pair ofreactive circuits, means for tuning one of said reactive circuits toresonance at a frequency below the mean frequency of said ldistortedwave energy, means for tuning the other of said reactive circuits toresonance at a frequency above the mean frequency of said distorted waveenergy, means for impressing the wave energy on said reactive cir-`cuits, a pair of detectors of substantial innite impedance each havinginput and output electrodes, means connecting the input electrodes ofeach of said detectors to the output of one f said filters whereby saidinput electrodes are excited by energy from the output of said filters,and a load circuit connected to the output of said detectors.

9. In a system for demodulating wave energy modulated in phase andfrequency, va pair of filters having opposed sloping characteristics,means for impressing vWave energy on the inputs of said filters, and apair of detectors of very high impedance connecting the outputs of saidfilters differentially, the very high impedance of said detectorsplacing minimum loading effect on'the outputs cf said filters.

10. In a system for demodulating wave energy modulated in\ phase orVinfrequency, a pair of filters of the sloping characteristic typeconnected back-toback, means for` impressing wave energy in phase on theinputs of said lters, a utilization circuit, and a pair of detectors ofinnite impedance connecting the outputs of said filters differentiallyand coupling the same to said utilization circuit, the infiniteimpedance of said detectors placing minimum load on said filters'.

11. In a system for demodulating wave energy modulated in phase atsignal frequency, a filter network including a piezo-electric crystal ina holderwith means to partially compensate for the capacity of theholder, said network having a highly selective characteristic such thatit attenuates the .side frequencies of phase modulated f wave energy ofthe proper mean frequency, and

`a sloping characteristic, a linear detector having an input vcoupled tosaid filter network and an from a selected amplitude as the frequency ofwave energy impressed on the input thereof varies about said particularmean frequency, means for Yimpressing frequency modulated wave energy ofsaid particular mean frequency on the input of said lter, a detector ofhigh impedance to the output of all frequencies coupled to said filter,and a circuit responsive to variations of modulation frequency connectedto said detector.

13. Ina systemfor demodulating wave energy modulated in phase at signalfrequency, means for attenuatingthe side frequencies of said wave energyto distort the modulated wave so that it has characteristics offrequency modulated wave energy, a pair of reactive circuits, means fortuning one of said reactive circuits to parallel resonance at afrequency above the mean frequency of said distorted Wave energy, meansfor tuning the other of said reactive circuits to parallel resonance ata frequency below the mean frequency of said distorted Wave energy,means for tuning said one of said circuits to series resonance at afrequency below the mean frequency of said .distorted wave energy,means' for tuning said other of said reactive circuits to seriesresonance at a frequency above the mean frequency of said distorted waveenergy, means for impressing the wave energy on said reactive circuits,a pair of detectors of substantial infinite impedance each having inputand output electrodes, means connecting the input electrodes of each ofsaid detectors to the output of one of said filters whereby said inputelectrodes are excited by energy from the output of said filters, and aload circuit connected to the output of said detectors.

MURRAY G. CROSBY.

